[OpenSCAD] Discuss manifoldness, co-incident faces edges etc

Jordan Brown openscad at jordan.maileater.net
Wed Nov 13 21:55:46 EST 2019


On 11/12/2019 6:25 PM, MichaelAtOz wrote:
> However, when I was learning, manifoldness, and self-intersections, was a
> hinderance, so having something to make life easier for beginner or casual
> users could be worthwhile.

I suspect that we're up to at least six variations of geometric
peculiarities that cause beginners problems:

  * Shared vertices or edges (or maybe faces)
  * Z-fighting with zero-thickness results from difference (and
    intersection?  and shared faces of different colors?)
  * Self-intersection
  * Polyhedra with winding errors
  * Blatantly malformed polyhedra (missing faces, et cetera)
  * The "convexity" parameter

I suspect that the answers to each are completely independent of one
another.  I suspect also that the class of "malformed polyhedra" might
well be very diverse.

> Can we help with these? 
> Can they be readily identified in more complex geometry?
>
> Would there be easily categorised resolution strategies/algorithms?

Unfortunately, I have no idea.  At least in a sense I expect them to all
be solvable because, to paraphrase Justice Stewart
<https://en.wikipedia.org/wiki/I_know_it_when_I_see_it>, I know the
right answer when I see the scenario.

But that doesn't mean that they are practically solvable.  They might
require expertise that is not available, or impractical amounts of work,
or impractical amounts of computation.

The only one that I have a hint on is "convexity"; setting it to 10
seems to fix all of my convexity problems and doesn't seem to noticeably
impact display times.  I can make some guesses on blatantly malformed
polyhedra, but I'm not sure.  (And I expect that my guesses wouldn't
catch all cases.)


---

With respect to the manifold-ness question, and in particular whether
it's useful and meaningful to represent objects with shared vertices or
shared edges... I think I've said all that I have to say and am
repeating myself, so I'll try to shut up.

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